The invention relates to a method of producing thixotropic metallic products by continuous casting of slabs or billets of circular, elliptical or polygonal cross-section.
A thixotropic metallic product is a metallic product which has a non-dendritic primary phase consisting particularly of dendrites which have degenerated into virtually spherical nodules.
These thixotropic products, when they are shaped, acquire advantages over conventional products:
the shaping energy is less, the cooling period is shortened, the contraction reduced, and mould and die wear is attenuated.
A number of patents disclose means of obtaining thixotropic metallic products:
U.S. Pat. No. 3,948,650 (equivalent to French Patent 2141979) describes a casting method which consists in raising the temperature of a composition until it reaches the liquid state, cooling to produce partial solidification and energetic agitation of the liquid-solid mixture to break up the dendrites and convert them to substantially spherical degenerated nodules representing up to about 65% by weight of the initial composition.
U.S. Pat. No. 4,434,837 describes an electromagnetic agitation apparatus applied to the continuous casting of thixotropic metal in which a stator having two poles creates a magnetic field which rotates in a plane at right-angles to the axis of the ingot mould and which is directed towards this axis. The interaction of this field with the current induced into the metal parallel with this axis generates electromagnetic forces situated in a horizontal plane and tangential in relation to the ingot mould, producing a level of shear of at least 500 sec.sup.-1.
U.S. Pat. No. 4,457,355 describes an ingot mould consisting of two parts of differing thermal conductivity and European Patent EP 71822 describes an ingot mould consisting of a succession of insulating and conductive metal plates.
U.S. Pat. No. 4,482,012 describes an improvement which consists of using an ingot mould consisting of two chambers connected to each other by a non-conductive seal, the first chamber acting as a heat exchanger and U.S. Pat. No. 4,565,241 advocates agitation conditions such that the ratio of the level of shear to the level of solidification is comprised between 2.10.sup.3 and 8.10.sup.3.